It is well known that the resistivity of many conductive materials change with temperature. Resistivity of a PTC conductive material increases as the temperature of the material increases. Many crystalline polymers, made electrically conductive by dispersing conductive fillers therein, exhibit this PTC effect. These polymers include generally polyolefins such as polyethylene, polypropylene and ethylene/propylene copolymers. Typically, polymers exhibiting PTC behavior will have temperature vs. resistivity characteristics such as those graphically illustrated in FIG. 1. At temperatures below a certain value, i.e., the critical or switching temperature, the polymer exhibits a relatively low, constant resistivity. However, as the temperature of the polymer increases beyond the critical temperature, the resistivity of the polymer sharply increases.
Devices exhibiting PTC behavior have been used as overcurrent protection in electrical circuits comprising a power source and additional electrical components in series. Under normal operating conditions in the electrical circuit, the resistance of the load and the PTC device is such that the current flowing through the device and the subsequent 1.sup.2 R heating of the device is small enough to allow the temperature of the device to remain below the critical or switching temperature. If the load is short circuited or the circuit experiences a power surge, the current flowing through the PTC device increases and its temperature (due to 1.sup.2 R heating) rises rapidly to its critical temperature. As a result, the resistance of the PTC device greatly increases. At this point, a great deal of power is dissipated in the PTC device. This power dissipation only occurs for a short period of time (a fraction of a second), however, because the power dissipation will raise the temperature of the PTC device to a value where the resistance of the PTC device has become so high, that the original current is limited to a negligible value. This new current value and corresponding high resistance of the PTC material is enough to maintain the PTC device at a new, high temperature / high resistance equilibrium point. The device is said to be in its "tripped" state. This negligible or trickle through current value will not damage the electrical components which are connected in series with the PTC device. Thus, the PTC device acts as a form of a fuse, reducing the current flow through the short circuit load to a safe, low value, when the PTC device is heated to the critical temperature range. Upon interrupting the current in the circuit, or removing the condition responsible for the short circuit (or power surge) the PTC device will cool down below its critical temperature to its normal operating, low resistance state. The effect is a resettable, electrical circuit protection device.
Generally, a separate discrete PTC device is required for providing protection to more than a single electrical circuit. In products having complex electrical circuitry having a large number of circuits and electrical components, e.g., an automobile or telecommunication equipment, the addition of numerous PTC devices often times consumes a limited amount of space allotted for the electrical circuitry of the product. Further, since each PTC device must be individually manufactured to include discrete elements (e.g., PTC element, terminals) the cost associated with providing electrical circuit protection for a plurality of circuits is increased.